Plastic lenses can be easily dyed with good quality and therefore they are dyed in various kinds of colors as customers desired. However, while such dyeing is usually performed at stores or factories, the actual situation is that determination of the dyeing conditions including kinds and concentrations of dyes, temperature of dye bath, immersion time in the dye bath and the like greatly relies on sense and experience of dyeing engineers.
For example, there has been known a color blending method using parameters generally referred to as dyers variables and known to artisans of dyeing and color toning. In this method, dyeing prescription is formulated by adjusting formulation ratio of dyes through comparison of concentrations of target color and sample color based on the knowledge of dyeing engineers that difference of color concentration corresponds to differences of dye concentrations and, when concentration of the sample color is adjusted to that of the target color, remaining color difference may be due to difference of formulation ratio of the dyes.
However, the above method has drawbacks that considerable skill is required to adjust the concentration of the sample color to that of the target color and that it is difficult to ultimately point to the target color because, when dye composition is changed to adjust the hue after the concentration is adjusted, the concentration may be changed again. Further, this method is mainly used for dying in the textile industry, painting using pigments in the coating industry and the like, but it is not suitable for dyeing of lenses where dyed color concentration changes depending on the immersion time in dye bath.
In the fields of dyeing textiles, painting, printing and the like, various kinds of techniques such as CCM (computer color matching) have also been known. The CCM method utilizes color correction equations represented by the following equations 1: ##EQU1##
In the equations, X, Y and Z are three stimulus values of color, C.sub.1, C.sub.2 and C.sub.3 are dye concentrations for dye formulation of the target color, .DELTA.X, .DELTA.Y and .DELTA.Z are differences between a sample color and the target color in the X, Y, Z coordinates and .DELTA.C.sub.1, .DELTA.C.sub.2 and .DELTA.C.sub.3 are necessary corrections for the dye concentrations.
The nine coefficients of from (.differential.X/.differential.C.sub.1) to (.differential.Z/.differential.C.sub.3) are correction factors representing variations of three stimulus values corresponding to changes of the dye concentrations.
The color correction equations represented by the above equations 1 are used for the final fine adjustment of dye formulation in most cases utilizing the CCM method. The color correction equations mentioned above have widely been used for dyeing in the textile industry, for painting using pigments and in the printing industry and the like. In these fields, since all of dyes or pigments are adsorbed or kneaded with dyeing objects, changes of colors and concentration with dyeing time are not considered at all. In addition, since all of the dyes are adsorbed on fibers or the like upon dyeing, interactions of dyes can be ignored and rough matching can function successfully. Therefore, the CCM method is used for the color correction only as an assistant means.
On the other hand, the conventional color correction equations, which do not take dyeing time into consideration, cannot be applied as it is to applications where dyeing concentration and hue may change with variation of dyeing time while dye concentrations are constant like in the field of lens dyeing.
As a method for color matching in dyeing lenses which does not require sense or experience of dyeing engineers, Japanese Patent Unexamined Publication No. Hei 4-226420 discloses a method for deciding dye concentrations of dye bath and dyeing time where the dyeing parameters are decided based on numerically represented spectroscopic characteristics of colored lenses.
However, this method has a drawback that dyeing rate and decolorization rate should be preliminarily determined for each dye. Further, this method is designed for rough matching where dyeing is performed based on the spectroscopic characteristics of colored lenses, but such rough matching is unsuitable for fine adjustment of color. Moreover, for usual colors, this method uses additional dyeing for color correction after dyeing in a dye bath of color close to the target color. This color correction is delicate operation and it is actually quite difficult.
Therefore, an object of the present invention is to provide a method which enables easier and more accurate decision of dye concentrations of dye bath and dyeing time for dyeing lenses, in particular, plastic lenses, in a target color.
Another object of the present invention is to provide a method for producing colored lenses, which utilizes the above method.